1. Field of the Invention
The present invention relates to a toner for use in a developer developing an electrostatic latent image formed by electrophotography, electrostatic recording, electrostatic printing or the like and to an image forming apparatus using the toner.
More particularly, the present invention relates to a toner for use in image forming apparatus such as copiers, laser printers and plain paper facsimile machines, which use a direct or indirect electrophotographic image forming method, and to a developer and an image forming apparatus using the toner.
In addition, the present invention also relates to a color toner for use in full color image forming apparatus such as color copiers, full color laser printers and full color facsimile machines, which use a direct or indirect electrophotographic developing method, and to a developer and an image forming apparatus using the color toner.
Further, the present invention relates to a toner for use in an electrophotographic full color image forming method in which a toner image formed on an image bearing member is first transferred on an intermediate transfer medium and the toner image on the intermediate transfer medium is second transferred on a receiving material, and to a method for manufacturing the toner and an image forming apparatus using the toner.
2. Discussion of the Background
Image forming methods in which an electrostatic latent image is visualized utilizing electrophotography, electrostatic recording or the like method are used for various fields.
For example, image forming methods using electrophotography include the following processes:    (1) a photoreceptor is charged (charging process);    (2) imagewise light irradiates the photoreceptor to form an electrostatic latent image thereon (light irradiating process);    (3) the electrostatic latent image is developed with a developer including a toner to form a toner image on the photoreceptor (developing process);    (4) the toner image is transferred on a receiving material optionally via an intermediate transfer medium (transfer process); and    (5) the toner image on the receiving material is fixed to produce a hard copy (fixing process).
As the developer, one-component developers constituted of only a non-magnetic or a magnetic toner, or two-component developers constituted of a toner and a carrier can be used.
Toners are typically prepared by the following kneading/pulverizing method:    (1) kneading a thermoplastic resin (i.e., a binder resin) and a pigment, optionally together with a release agent such as waxes and a charge controlling agent, upon application of heat thereto;    (2) cooling the kneaded mixture;    (3) pulverizing the mixture;    (4) classifying the pulverized mixture to prepare a mother toner;    (5) optionally mixing the mother toner with an external additive such as inorganic or organic particulate materials to improve the fluidity or cleanability of the resultant toner, resulting in preparation of a toner.
The toners prepared by such a kneading/pulverizing method have an irregular form. It is possible to slightly change the form of such toners by changing the raw materials and/or pulverizing conditions, however, it is impossible to freely control the form and structure of the surface of the resultant toners.
In addition, it is hard to prepare a toner having a particle diameter distribution narrower than ever because current classifiers has a limited classification ability. Namely, when it is tried to prepare such a toner, a problem occurs in that the manufacturing cost seriously increases. In addition, it is hard to prepare a toner having such a small average particle diameter as not greater than 6 μm in view of yield, productivity and manufacturing cost of the toner.
The toners having an irregular form have a drawback in that the toner particles have different charging properties, and thereby the toner particles have different developing abilities because particles of a one-component developer having an irregular form have different contact areas (i.e., different adhesion) with the developing roller used and toner particles included in a two-component developer have different contact areas (i.e., different adhesion) with the carrier included in the two-component developer.
In addition, toner particles having different particle diameters have different charge quantities, and thereby the toner particles have different developing abilities.
When such toner particles having different developing abilities are present in a developer, toner particles which have a high developing ability are selectively used for developing, i.e., toner particles having a low developing ability tend to remain in a developing device, resulting in change of the developing properties of the toner (i.e., developer).
Similarly, when a toner image is transferred onto a receiving material, the toner image includes toner particles having different transferring abilities, and thereby image defects such as toner scattering tend to be produced.
In addition, when a toner is prepared by internally adding a-release agent such as waxes, the release agent tends to be mainly present in surface portions of the toner particles, depending on the affinity of the release agent with the resin included in the toner particles. In particular, when a resin which has an elasticity because of including high molecular weight components and which is hard to pulverize is used in combination with a brittle wax such as polypropylene, the surface portions of the resultant toner particles tend to include the wax in an amount greater than that inside of the toner particles.
The toner in which a release agent is present in the surface portions at a high content has advantages such that the toner particles tend to be easily released from fixing devices and toner particles remaining on a photoreceptor can be easily removed. However, the toner has drawbacks in that the toner contaminates the developing rollers, photoreceptors and carriers used and thereby the reliability of the image forming apparatus deteriorates.
In attempting to solve the problems of the kneading/pulverization methods, suspension polymerization methods have been proposed. Since the suspension polymerization methods do not include the kneading and pulverization processes, the methods have advantages in that production energy can be saved, production time can be shortened and yield can be increased, resulting in decrease of manufacturing cost.
In addition, the toners prepared by such suspension polymerization methods have relatively narrow particle diameter distribution compared to the toners prepared by the kneading/pulverization methods. Further, it is possible to include a wax inside toner particles, resulting in improvement of the fluidity of the resultant toner.
Thus the suspension pulverization methods have various advantages. Therefore the pulverization methods attract attention now, and researches in binder resins and polymerization methods have been vigorously performed.
However, the suspension polymerization methods have drawbacks. For example, toner particles prepared by the suspension polymerization methods have almost a true spherical form because surface tension acts on particles in a polymerization process. Toners having a spherical form have good charge stability and transferability but have poor cleanability, i.e., toner particles remaining on the surface of an image bearing member can be hardly removed, and thereby problems such that the resultant images have background fouling and uneven image densities because the developing density cannot be controlled due to the remaining toner particles. Specifically, when toner particles remain on the surface of a photoreceptor, the surface potential is mistakenly measured, and thereby the next image is developed under improper developing conditions, resulting in formation of an image having an undesired image density.
With respect to the toner form of the toners prepared by such suspension polymerization methods, Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 11-149177 discloses that when a toner having a form coefficient SF-1 not greater than 110, it becomes hard to remove toner particles remaining on an image bearing member, resulting unsatisfactory cleaning. JOPs 08-44111 and 08-286416 have disclosed suspension polymerization methods. When a toner is prepared by such a suspension polymerization method, it is necessary to control the particle diameter of the suspended particles so as to fall in a proper range. Therefore, it is necessary to strongly agitate the dispersion at a high speed such that the toner constituents are finely dispersed in a dispersion medium. However, in general a release agent and a monomer which is to be polymerized to form a binder resin have different viscosity and have poor compatibility with each other. Therefore to finely disperse the toner constituents in the suspension process is very difficult. As a result, a number of toner particles not including a wax (i.e., a release agent) are included in the resultant toner, namely a wax is unevenly present in toner particles, and thereby the resultant toner has poor charge stability and cleanability.
JOP 05-34979 discloses a toner for use in an image forming method in which a toner image formed by a developer bearing member of a two component developing device having functions of developing, transferring and cleaning is transferred on a receiving material, wherein the toner has a concavo-convex surface. Toner particles having concave and convex on their surface necessarily have projections on their surface. When such toner particles are used in combination with a carrier to perform two-component developing, the projections are abraded, resulting in change of the form of the toner particles, and thereby the cleanability of the toner deteriorates.
In addition, when a toner is prepared by suspension polymerization, the toner tends to include residues such as monomers, e.g., styrene monomers and acrylic monomers, which are not preferable in view of environmental pollution. Further, the toner has good fluidity and a problem in that the toner adheres to the photoreceptor used can be avoided because a release agent such as waxes is included in the toner particles. However, since a release agent is included in the toner particles, the toner has poor fixing efficiency because fixing energy is needed more than in the case of using a toner which is prepared by the kneading/pulverization method and in which the release agent mainly located at the surface portions of the toner particles.
When the addition amount of a release agent is increased or the particle diameter of a release agent dispersed in toner particles is increased in attempting to improve the fixability, the transparency of the resultant toner deteriorates and thereby the color tones of color images formed by projecting color toner images formed on an OHP sheet deteriorate.
Methods for preparing a toner utilizing polymerization include not only the suspension polymerization methods mentioned above but also emulsion polymerization methods and solution-suspension polymerization methods, in which the form of the resultant toner can be relatively easily changed into a sub-spherical form, i.e., a form which is similar to true spherical form but is slightly deformed, such as orbital forms, and spherical forms having a rough surface.
When emulsion polymerization methods are used, it is difficult to perfectly remove monomers (e.g., styrene monomers) emulsifiers and dispersants. This is a large problem in view of environmental protection. In addition, toner particles prepared by an emulsion polymerization methods typically have a concavo-convex surface to prevent the cleaning problem mentioned above. In this case, an external additive (such as silica) added thereto has poor adhesion to the concave surface or the external additive adhered to the convex surface is transferred on the concave surface, resulting in decrease of the adhesion of the external additive to the toner particles, and thereby the external additive is released from the toner particles. Thus, problems in that the photoreceptor and fixing roller used are contaminated by the toner occur.
When solution-suspension methods are used, it becomes possible to use polyester resins typically having a good low temperature fixability. However, the methods have the following drawbacks:    (1) in order to prepare a toner for use in oil-less fixing methods or to widen the-releasing temperature range, the quantity of the high molecular weight (about not less than 100,000) components in the resultant polymer toner has to be controlled, which is very difficult; and    (2) when a toner is prepared, a high molecular weight component is added in a process in which a resin and a colorant are dissolved or dispersed in a solvent, resulting in increase of the viscosity of the dispersion (or solution), and thereby it is difficult to stably produce a toner.
These drawbacks have not yet been remedied.
JOP 09-15903 discloses a toner which is prepared by a solution suspension method and which has a spherical form, wherein the surface of the toner particles has concave and convex in attempting to improve the cleanability of the toner. However, the concave and convex are formed irregularly, and thereby the toner has poor charge stability. In addition, the content of the high molecular weight components is not controlled, and therefore the qualities of the toner such as durability and releasability are not satisfactory.
JOP 11-149180 discloses a toner which is prepared by reacting a prepolymer having an isocyanate group with an amine in an aqueous liquid to perform an elongation reaction and a crosslinking reaction, resulting in formation of toner particles. Small toner particles can be made by this method but the form of the toner particles is hardly controlled because the way to control the spherical degree of the toner particles is not described therein.
Electrophotographic image forming methods typically include a developing process, a transfer process, a cleaning process and a fixing process. In addition, there are color image forming methods in which plural color toner images formed on a photoreceptor or plural photoreceptors are transferred on an intermediate transfer medium to form a (full) color image and the color image is then transferred on a receiving material. When polymerization toners (i.e., toners having a spherical form) are used in such color image forming methods, toner particles remaining on the intermediate transfer medium cannot be easily removed although the toners have good transferability. Therefore, the polymerization toners are needed to be improved.
Because of these reasons, a need exists for a toner which can produce high quality images even when the toner images are fixed at a low temperature by an oil-less fixing method while having good cleanability and which can be stably manufactured.